1. Field of Invention
This invention relates to a piezoelectric ceramic hydrostatic sound sensor or transducer having one or a plurality of voids and to a method for making such a transducer.
2. Description of the Prior Art
Conventional piezoelectric ceramic hydrophones employ relatively incompressible materials such as lead zirconate titanate (PZT) having the general formula (PbO)(ZrO.sub.2).sub.0.52 (TiO.sub.2).sub.0.48 ; PZT doped with 6-15% lanthanum oxide, La.sub.2 O.sub.3 (PZLT); barium titanate, BaTiO.sub.3 ; lead zinc niobiate, (PbO)(ZnO)(Nb.sub.2 O.sub.5); and lead magnesium niobiate, (PbO)(MgO).sub.0.33 (Nb.sub.2 O.sub.5).sub.0.67 ; The electromechanical response of ceramic transducers to hydrostatic pressure variations is only a fraction of their uniaxial electromechanical sensitivity because, due to their Poisson ratio, the lateral force components due to hydrostatic pressure tend to cancel out the axial compression of the material, thereby reducing the electromechanical response to hydrostatic pressure.
Improvements in the electromechanical response of ceramic transducers to hydrostatic pressure have been achieved by the provision in the ceramic transducer of voids or pores. Randomly spaced voids provide some improvement in electromechanical response but tend to weaken the ceramic structure, making it susceptible to breaking. Regularly-spaced voids of uniform dimensions provide improved electromechanical response without the loss of mechanical strength and without increased susceptibility to breaking.
U.S. Pat. No. 4,683,161 provides ceramic bodies with ordered pores or voids and a method of making such ceramic bodies. The method employs thermally fugitive materials to create voids in the ceramic material.
U.S. Pat. No. 4,353,957 provides a method for forming monolithic ceramic capacitors having ceramic dielectric insulators. Thermally fugitive material is used to create voids in the ceramic. These are filled with metal to create capacitor plates.
U.S. Pat. No. 4,617,707 provides a method for manufacturing ultrasonic antenna arrays by laminating alternate layers of green ceramic and heat-fugitive filler material and subsequently removing such filler material by heating.
U.S. Pat. No. 4,753,964 provides a method of manufacturing a multilayered ceramic substrate having embedded and exposed conductores for mounting and interconnecting electronic components. A pattern of solid, nonporous conductors is attached to a backing sheet, transferred to a green ceramic sheet and sintered.
U.S. Pat. No. 4,806,295 provides a method of preparing ceramic monolithic structures with internal cavities and passageways by forming individual layers of ceramic by cutting and punching, stacking these layers and sintering.
U.S. Pat. No. 4,867,935 provides a method of preparing a dielectric ceramic composition containing hollow microspheres which can be cast on a substrate in the form of a tape or sheet for multilayer circuits.
U.S. Pat. No. 4,885,038 provides a method for producing multilayered ceramic structures having copper-based conductors therein.